The present application relates to vehicle braking systems and, more particularly, vehicle braking systems that utilize a hydraulic master cylinder.
It is common for vehicles such as automobiles to be operated on unlevel terrain. When a vehicle equipped with a manual transmission begins to move on an uphill grade, the driver must release the vehicle's brake pedal while at the same time releasing the clutch pedal and pressing on the accelerator pedal. If the hill is particularly steep, the vehicle may roll backward if the brake pedal is released before the clutch is engaged. This can cause the vehicle to collide with objects and/or a loss of control. Even properly executed, this process of “slipping the clutch” contributes to driver fatigue and wearing of the vehicle's clutch and brake.
Likewise, vehicles utilizing automatic engine-stopping technology also encounter difficulties when operating on unlevel terrain. These “idle-stop” vehicles are adapted to turn off the engine when the vehicle is stopped (such as at a traffic light) to conserve fuel and reduce emissions. The engine automatically restarts when the driver presses on the accelerator. To enable restarting, the transmission is automatically disengaged while the engine is off, and is automatically re-engaged only after the engine is running again. While the transmission is disengaged, the vehicle is free to roll forward or backward. Since there is an inherent lag between the time the engine is started and the time the transmission is re-engaged, there is a need for a means to keep an idle-stop vehicle from rolling when started from a stopped position on unlevel terrain.
It is desirable to provide an automatic braking system for use with idle-stop vehicles and vehicles having a manual transmission. Various braking systems that utilize a hydraulic master cylinder are preferably adapted for use in an automatic braking system due to the prevalence of hydraulic brakes. Examples in the art include Delphi TCS6, DBC7, Smartboost and Eboost systems. However, available hydraulic braking systems all suffer from one or more drawbacks, such as noise, limited brake holding time, limited brake pressure at altitude, and limited brake pressure with the engine off. These drawbacks limit their use as automatic brakes.
Accordingly, there is a need for a low-noise automatic brake holding system that is capable of holding the brake for a sufficient period of time, operating at higher elevations and providing sufficient brake pressure with the engine off. There is a particular need for an automatic brake holding system having a hill-holding capability that is able to automatically keep the brakes applied after a stop, such that the driver need not keep his or her foot on the brake, and then gradually releases the brakes when the accelerator is engaged, enabling a smooth start and preventing the vehicle from unintentionally rolling when operated on unlevel terrain.